Method of remelting and refining magnesium and light metal alloys containing magnesium



Patented Feb. 8, 1938 UNITED STATES PATENT OFFIC Adolf Beck, Bitterfeld,Germany, assignor, by

mesne assignments, to Magnesium Development Corporation, a corporationof Delaware No Drawing. Application July 10, 1936, Serial No. 90,006. 1In Germany July 19, 1935 7 Claims.

This invention relates to a method of re-melting and refining magnesiumand light metal a1- loys containing magnesium, more particularly highpercentage magnesium-base alloys.

It has of late frequently been asserted that the saline melts employedfor re-melting and refining magnesium and high percentage magnesium-basealloys, and containing, besides chlorides of the alkaline-earth metals,andon occasion of the alkali metals, additions of thickening orinspissating substances such as metal oxides or fluorides, melt only attemperatures at which the metal to be treated is already in a moltenstate, and that, in consequence, the tendency of the molten metal torelatively extensive oxidation cannot be entirely suppressed by thefiuxing agent, which was supplied in the form of powder,

in the bottom of the crucible, or strewn over the metal to be melted,since at the melting temperature of the metal said fiuxing agent wasstill unmolten and thus incapable of protecting the metal from oxidationby the formation of a coherent layer.

In order to obviate this drawback, it has been proposed to employ fluxesfusing below the melting point of the metal under treatment, theintention being for the fiux, by melting first, to

form a protective cover on the metal which has not yet melted, or is incourse of melting. However, in the case of these fluxes, which containno inspissating agents, or only small amounts thereof, the thickeningeflect obtainable by the addition of such inspissating agents is ofcourse absent, that is to say, owing to their high fluidity,

or low surface tension at higher temperatureeven at such as are onlyabout IUD-150 C. above the melting point of the metals undertreatment-it is impossible to obtain a clean separation of the flux fromthe metallic regulus after the treatment is finished; and there istherefore always a risk of traces of fiux being entrained by, orincluded in, the metal, on pouring, such traces afterwards giving riseto the corrosion phenomena which, for a long time, made the 45commercial employment of magnesium. or its alloys impossible.

Nevertheless, by suitably modifying the known process of re-melting andrefining light metals, with the aid of chloride fluxes containinginspis- 59 sating or thickening agents, it is possible, on the one hand,to prevent any extensive oxidation of the metal during re-melting and,on the other, to secure a clean separation of the flux from the treatedmetal, under all conditions.

According to the present invention the known process of melting andrefining magnesium and light metal alloys containing magnesium ismodified with a view to ensuring an efiicient protection, fromoxidation, of the metal to be treated, during the melting stage by thepresence of an easily fusible flux, whilst at the same retaining theadvantages resulting from the use of inspissated fluxes, namely completeremoval of the flux from the metal after completion of the refiningoperation. I

To this end, according to the invention, the metal under treatment isfirst melted down in the presence of a thinly fluid, i. e.,non-inspissated, flux containing one or more chlorides and having amelting point which is not substantially higher than that of the metal,and which on occasion, may be substantially below the latter.-Substances which have been found particularly suitable for this purposeare anhydrous carnallite, or mixtures still richer in potassium chloridesuch as for example, 2KCLMgCl2, having a melting point of 580 C. orless, that is to say, at least about 70 C. below the melting point ofpure magnesium. The desired efiect may, however, also be obtained withpure anhydrous magnesium chloride, containing additions of chloridesother than potassium chloride, tending to lower the melting point.

In carrying out the invention, it is immaterial whether, by using asomewhat larger quantity of the flux, the metal melts down into a sumpof flux in the bottom of the crucible, or whether the fiux be strewn aspowder, over the metal to be melted, or added by degrees in accordancewith the progress of the melting operation. In any event, the readilyfusible fiux will afiord extensive protection to the metal againstsubstantial oxidation during the melting operation. As soon as asubstantial sump of molten metal has formed, further portions of fluxmay be applied, as a mobile protective coating, to the surface of themetal, in order to protect this portion of the metal from any extensiveoxidation.

When the whole of the metal under treatment is in a molten state, the'melt is energetically stirred in the usual manner, in order to enablethe flux to take up any impurities contained in the metal. Thisoperation can be carried -out, without substantial oxidation of themetal, at temperatures below about 800 C. During this cleansing action,the known inspissating or thickening agents may be added to the metalbath, in suitable amounts for inspissating the flux already present, oralternatively, suitable amounts (according to circumstances) may beadded, of a flux of fundamentally the same composition as that of theknown fluxes containing inspissating or thickening agents, butpreferably with a larger proportion of these latter than usual, in orderthat, in being contacted with the mobile flux already present in themetal bath, the newly added flux may also have an inspissating eifectthereon. The known mixtures of magnesium chloride with magnesiumfluoride, calcium fluoride and/or magnesium oxide, or of calciumchloride with magnesium fluoride, are specially suitable for thispurpose.

After the stirring has been completed, which, in the case of smallunits, usually does not take any longer than about 1 minute, but may becontinued for a considerable time, or be frequently repeated, ifnecessary, in accordance with the amount of impurities contained in themetal, the melt is covered, in known manner, with a protective layer ofa flux containing a normal proportion of inspissating agents, and isthen heated to considerably abovethe pouring temperature for the purposeof separating the final traces of agglomerated flux from the metallicregulus. The metal is thereupon poured, on occasion after cooling topouring temperature, and the coherent solid covering of flux is heldback by suitable means such as rakes or the like.

The fluxes employed in carrying out the invention may consist merely ofmixturesuof the ingredients. Sometimes, however, it is preferable topre-melt the ingredients down together and to crush the mass aftersolidification, because this method ensures more intimate mingling ofthe metal or alloy.

components. If desired, salts of a metal which is already present in thealloy, may be incorporated with the flux, in known manner, in order toprevent the said metal being washed out during the treatment with theflux. The fluxes may moreover contain additions which become reduced bythe metallic melt and enter into the Example A cast-iron crucible isfirst strewn with a thin fluxing layer consisting of anhydrouscarnallite, and is then charged with about 30 kgs.of a high percentagemagnesium alloy, in the form of ingots, and heated. Directly incipientmelting is observable, further quantities of the same flux are strewn insmall portions over the still solid lumps of metal, and, after a metalsump has formed, the surface of this also is covered with a littleanhydrous carnallite. The total amount of flux used, up to the end ofthe melting process, is 200 grammes. T

When the temperature reaches 720 C. the metal is thoroughly stirred forabout a minute with a stirring rod, and, at the same time, a. totalamount of 250 grammes of a fiux, consisting of:

4 Per cent Anhydrous MgCh 70 MgO 10 Cal: 20

Per cent MgCla. MgO 10 CaFz 10 which soon sinters in the form of acoherent,

viscous layer. Under this 1: er, the contents of the crucible are heatedat about 850 C. for a short time, then cooled to the pouring temperatureof 760 C. and poured in known manner.

I claim: I

1. A method of re-melting and refining magnesium and light metal alloyscontaining magnesium and more particularly high percentagemagnesium-base alloys, which comprises melting the metal in the presenceof a thinly fluid saline flux having a melting point not substantiallyexceeding the melting temperature of the metal, then increasing thetemperature of the metal, then stirring the metal at such elevatedtemperature with the said flux while adding a thickening agent in aquantity sufiicient to cause inspissation of the flux already present,so as to cause said flux to combine with said inspissating agent andwith the non-metallic impurities contained in the metal, and finallypouring said metal.

2. A method of re-melting and refining magnesium and light metal alloyscontaining magnesium and more particularly high percentagemagnesium-base alloys, which comprises melting the metal in the presenceof a thinly fluid saline flux having a melting point not substantiallyexceeding the melting temperature of the metal, then increasing thetemperature of the metal, then stirring the metal at such elevatedtemperature with the said flux while progressively adding portions of athickening agent in an aggregate quantity suflicient to causeinspissation of the flux already present, so as to cause said flux tocombine with said inspissating agent and with the non-metallicimpurities contained in the metal, and finally pouring said metal.

3. A method of re-melting and refining magnesium and light metal alloyscontaining magnesium and more particularly high percentagemagnesium-base alloys, which comprises melting the metal in the presenceof a thinly fluid fiux' containing a substantial proportion of magnesiumchloride and having a melting point not substantially exceeding themelting temperature of the metal, then increasing the temperature of themetal, then stirring the metal at such elevated temperature with thesaid flux while adding at least one thickening agent selected from thegroup of metal oxides and metal fluorides in a quantity sufficient tocause inspissation of the flux already present, so as to cause said fluxto combine with said inspissating agent and with the non-metallicimpurities contained in the metal, and finally pouring said metal.

4. A method of re-melting and refining magnesium and light metal alloyscontaining magnesium and more particularly high percentagemagnesium-base alloys, which comprises melting the metal in the presenceof a thinly fluid fiux containing a substantial proportion of magnesiumchloride and having a melting point not substantially exceeding themelting temperature of the metal, then increasing the temperature ofmagnesium-base alloys, which comprises melting the metal in the presenceof a thinly fluid flux containing a substantial proportion of magnesiumchloride and having a melting point not substantially exceeding themelting temperature of the metal, then increasing the temperature of themetal to at least about 720 ,C., then stirring the metal at suchelevated temperature with the said flux while adding at least onethickening agent selected from the group of metal oxides and metalfluorides in a quantity suflicient to cause inspissation of the fluxalready present, so as to cause said flux to combine with saidinspissating agent and with the non-metallic impurities contained in themetal, and finally pouring said metal.

6. A method of re-melting and refining magnesium and light metal alloyscontaining magnesium and more particularly high percentagemagnesium-base alloys, which comprises melting the metal in thepresenceof a thinly fluid flux containing a substantial proportion of magnesiumchloride and having a melting point not substantially exceeding themelting temperature of the metal, then increasing the temperature of themetal, then stirring the metal at such elevated temperature with thesaid flux while adding an inspissating agent comprising magnesium oxidein a quantity sumcient to cause inspissation of the flux alreadypresent, so as to cause said flux to combine with said inspissatingagent and with the non-metallic impurities contained in the metal, andfinally pouring said metal.

7. A method of re-melting and refining magnesium and light metal alloyscontaining magnesium and more particularly high percentagemagnesium-base alloys, which comprises melting the metal in the presenceof a thinly fluid saline flux having a melting point not substantiallyexceeding the melting temperature of the metal, then increasing thetemperature of the metal, then stirring the metal at such elevatedtemperature with the said flux while adding a thickening agent in aquantity sufficient to cause inspissation of the flux already present,so as to cause said flux to combine with said inspissating agent andwith the non-metallic impurities contained in the metal, establishing aprotective layer of inspissated saline flux on the surface of the moltenmetal, heating the metal to a temperature above about 800 C., andfinally allowing the metal to cool to pouring temperature. ADOLF BECK.

